Essentially, a decision-maker that has failed to follow procedural requirements leaves itself more vulnerable to judicial review challenge than a decision-maker that complies with all relevant requirements. Nevertheless, the prospects of a claimant succeeding with a challenge on the grounds that the decision-maker failed to follow proper procedure depends on the facts of each case, particularly the nature of the requirement that has not been complied with and the consequences of that failure.
In considering whether a decision ought to be quashed for failing to comply with a particular procedure, the key question that the Court will consider is whether the purpose (or intention) of the legislation that sets that requirement is that any decision that is made in contravention of that procedural requirement is invalid. (R v Clarke  1 WLR 338.)
For the more technical procedural requirements (e.g. a requirement to take a particular step within specified timescales, or to use a particular form), the Court will also consider matters such as:
- whether there has been substantial compliance with the procedural requirement, and whether substantial (not strict) compliance would fulfil the procedural requirement set out in legislation
- whether the non-compliance may be waived (i.e. put to one side by the person that is affected by the non-compliance)
- if the non-compliance may not be waived, what is the consequence of non-compliance. (R v Immigration Appeal Tribunal ex p Jeyeanthan  1 WLR 354).
If the Court is of the opinion that a breach of a procedural requirement would defeat the purpose of the legislation that sets the requirement, it is likely that the decision will be quashed.
However, where the procedural requirement is a technical one, and the non-compliance is relatively minor (e.g. if a requirement to publish a notice is complied with, but is complied with one day late), or where there has been substantial compliance, or where the Court is of the view that the non-compliance does not defeat the purpose of the legislation, a decision may not necessarily be quashed.
Where the procedural requirement is more substantive (e.g. a requirement to consult with interested parties, or to notify affected parties of a decision), it is more likely that that the Court will consider that any non-compliance will defeat the purpose of the legislation and is more likely that the decision will be quashed.
Nevertheless, as with all other grounds for judicial review, section 31(2A) of the Senior Courts Act 1981 sets out that the Court must refuse to quash the decision (or award any other relief) if it appears to it to be highly likely that the outcome for the person affected by the decision would not have been substantially different if the decision-maker had not failed to comply with the procedural requirement in question, unless it considers there to be exceptional public interests reasons for awarding the remedy.